Differences in focal lengths

[dreaminblue]

I'm confused! I've been using Nikon D80 and Nikkor 28-200 lens for years so I've never paid much attention to what the 'real' focal length is and just assumed for a D80 I multiply by 1.5.

Recently I upgraded my lenses so now I use a Tamron 18-270 as my main walkabout lens and a Sigma 70-200 f2.8 EX DG for when I want something a little better. I naturally assumed the Tamron had a longer focal range but last night I realised that from 1 metre away I was able to zoom in a lot closer with the Sigma at 200mm than the Tamron at 270mm! I assumed that the Tamron was for a crop camera so the 18-270 has probably already been converted to give a true focal range. The 70-200 is probably for a full frame camera so multiply that by 1.5 to give a 100-300 zoom. So now my Sigma has the longer focal length so now it makes sense as to why that allowed me to zoom in further, right? Not quite, as the amount more that I was able to zoom in was way beyond the extra 30mm I was getting. The Sigma was giving more like 400mm or even more.

Can anyone de-confuse me? :shrug:
Cheers,
Terry

 

[futureal33]

Terry,
I think you're getting yourself confused about focal lengths and sensor crop.

Basically you have a crop sensor, which as you say, means any lens you put on that camera is effectively multiplied by 1.5x

The same applies for any lens you put on there, regardless of whether that lens was made for full frame or crop frame cameras.

However, you must be mistaken about being able to "zoom in" further with the 70-200 vs the 18-270mm because the latter has an extra 70mm "zoom" to it.
Im not sure of the minimum focus distances of these 2 lenses, but that may be part of what you are experiencing. One may not be able to focus properly at just 1 metre

Go back and try the same thing again, I think you will find that your Tamron 18-270mm "zooms in" further than the Sigma, but obviously the Sigma should have the upper hand in terms of Image Quality

HTH

 

[Martyn...]

The focal length of any lens is at infinity, some zoom lenses will not achieve their maximum focal length at close distance

This is part of the compromise the optical engineer designed into the lens in order to get it to work.

I suspect that your 18-270 is showing this, especially at 1.00m.

The latest Nikkor 70-200 is noted for it.

From Ken Rockwell's notes on the 70-200

At infinity, you get the full 70-200mm focal length range.

Nikon used an optical trick common in wider-range zooms, like the 18-200m VR and Canon's 28-135mm IS, to get extra-close focusing: the actual focal length gets shorter at closer focus distances.

Every full-time pro I interviewed prefers a physically shorter focus distance in exchange for a slightly shortened maximum focal length. Why? A pro usually only carries this lens and a wide zoom. If he's shooting in confined quarters, he may not have five feet between him and his subject.

The few extra inches of close-focus will make the difference between getting the head shot with this tele, or having to use his wide zoom at 35mm instead, which is not a pretty picture.

The pro can always crop to make up the slightly shortened focal length, we have way too many pixels today, but you can't use a shot that's not in focus.

That said, this 70-200mm VR II is the closest focusing f/2.8 tele zoom ever from Nikon, and also the most foreshortened at close focus distances.

At ten feet (three meters) and the 200mm setting, this 70-200mm VR II gives about the same field of view as other 80-200mm zooms set about halfway between their 135mm and 200mm settings.

At 5 feet (1.5 meters), the 200mm setting is similar to only a 135mm lens.

 

[hoolio]

That's actually really interesting, didn't know that.

 

[Martyn...]

That's actually really interesting, didn't know that.

Thom Hogan covers the same issue LINK, scroll down the page a bit.

 

[dreaminblue]

Thanks Martyn, I never knew that! Zoom lenses state their aperture at min/max focal lengths but it seems that they should all give two sets of focal ranges as well, one for minimum focus distance and one for infinity! I'd assume that as my 70-200 does macro it probably stays more constant to 200mm at minimum focus but the 18-270 is probably nowhere near 270mm at minimum focus.

Almost can't wait to get home to test them again (how sad!). Love learning something new!
Cheers

 

[MartynK]

Terry, the focal length remains the same, irrespective of whether you're using a full frame camera or a crop body. A 70-200mm is still a 70-200mm. It's the field of view that changes, reducing by 1.6 or 1.5 (Canon and Nikon), so the subject fills more of the frame, assuming the same distance. Theoretically, you can produce the same result by cropping a full frame image by the same proportion, but of course you will lose pixels.